Astronomers using the James Webb Space Telescope (JWST) have confirmed that the ultra-hot exoplanet WASP-121 b possesses distinct, asymmetrical atmospheric conditions between its dawn and dusk regions. By observing infrared starlight filtered through the planet’s atmosphere during transit, researchers at the Max Planck Institute for Astronomy (MPIA) identified that the evening side is significantly hotter and chemically different from the morning side, providing the first clear evidence of longitudinal atmospheric variations on a gas giant.
Why are the morning and evening sides of WASP-121 b different?
The asymmetry is driven by powerful, planet-wide winds that shift heat from the day side toward the night side. According to Cyril Gapp of the MPIA, these winds move eastward in the direction of the planet’s rotation, effectively “piling up” heat on the evening terminator. Because the atmosphere expands as it warms, the evening side presents a larger cross-section to incoming starlight, leading to increased radiation absorption compared to the cooler morning terminator.
WASP-121 b is a “tidally locked” planet, meaning one side constantly faces its host star while the other remains in perpetual darkness. Day-side temperatures reach approximately 2,770 Kelvin (2,500 degrees Celsius), while night-side temperatures drop to around 1,000 Kelvin (725 degrees Celsius), according to co-author Tom Evans-Soma of the University of Newcastle.
How does JWST map the atmosphere of a distant planet?
Researchers utilize the planet’s own rotation during its transit across the host star to scan the atmosphere longitude by longitude. As WASP-121 b moves, it rotates by roughly 30 degrees, allowing the JWST’s NIRSpec instrument to capture separate spectral data for the morning and evening regions. By analyzing how light absorption changes over time rather than averaging the data into a single signal, scientists can distinguish the chemical composition of specific zones, such as the varying levels of carbon monoxide and water vapor.
What role do clouds play in these extreme atmospheres?
While current computer simulations accurately predict the general temperature-based asymmetry, the observed effects are stronger than the models suggest. Researchers suspect that clouds—likely composed of minerals like silicates rather than water—may be cooling the morning terminator. According to the study, when these cloud effects are incorporated into simulations, the data aligns more closely with real-world observations. However, modeling cloud formation in such extreme, rapidly changing environments remains a significant challenge for modern astrophysics.
What are the future trends for exoplanet research?
The success of the “longitude-by-longitude” mapping technique on WASP-121 b signals a shift toward 3D atmospheric modeling of exoplanets. Astronomers have already identified other ultra-hot gas giants suitable for similar observations. By comparing a larger sample of planets, researchers aim to move beyond simple 1D averages to create detailed, three-dimensional maps of weather patterns on worlds light-years away. This method will likely become a standard tool for the JWST, which is operated by NASA, ESA, and the CSA.
When reading about exoplanet data, look for the distinction between “averaging” and “time-resolved” data. Averaging is traditional, but time-resolved observations—like those used in the WASP-121 b study—are the key to unlocking the complex, asymmetrical weather patterns of distant worlds.
Frequently Asked Questions
- What is a terminator on an exoplanet?
The terminator is the boundary zone that marks the transition between the permanent day side and the permanent night side of a tidally locked planet. - Why does water disappear on the hot side of WASP-121 b?
Scientists interpret the decrease in water molecules as a result of extreme heat in the upper atmosphere, which is sufficient to break water molecules apart into their constituent elements. - Could these findings apply to planets like Earth?
No. WASP-121 b is an “ultra-hot Jupiter” gas giant. Its conditions, including temperatures exceeding 2,500 degrees Celsius, are vastly different from the temperate, rocky environment of Earth.
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